The Green Man who lives in the roof of Norwich Cathedral’s cloisters.
Photograph: Alamy

The temple at Hatra in Iraq, destroyed last year, featured a carved figure with an acanthus-leaf beard, a possible prototype of the Green Man. On corbels and ceiling bosses and misericords closer to home, the Green Man appears as either a human tormented by nature or a species of woodland sprite, the Jack in the Green of May Day parades. According to Nina Lyon, in her riotously fecund book about “hunting” the Green Man, “he is a sort of forest-god, an emblem of the birth–death–rebirth cycle of the natural year … a reminder of the superior force of Nature over human enterprise”.

Among the hundreds of depictions of the figure in the churches and cathedrals of western Europe, rarely is he serene or smiling: as Kathleen Basford points out in a study of Green Man imagery published nearly 40 years ago, he is “bellicose, morose, even comatose, but seldom jocose”.

We don’t hear much nightingale song in Lyon’s book, and she has little patience for the kind of chirpy environmentalism that makes gods of dolphins and pandas – or for the elevation of humans to the status of guardians of the world. What is needed, she writes, is “a renunciation of the Enlightenment idea of humans being special”. The way to do this, she decides, is to “rekindle a Green Man cult”. After a downpour, “something about the surfeit and force of elements” prompts her to visit the church in Kilpeck, Herefordshire, with its Green Man carvings. While the book’s heartland is the ancient Welsh kingdom of Archenfield, close to Lyon’s home near Hay-on-Wye, she traces the Green Man’s complex root system from neopagan Cornish festivals to the forests of south-west Germany. The book begins in urgency, then, as a physical quest, but it becomes increasingly an account of a philosophical, even political, mission, if not exactly a personal one. “I was not in a mood for finding God or redemption.”

This isn’t a nature memoir, though we get glimpses of Lyon’s life in rural Wales, her children and her ex, and her father digging his allotment in leafy Chislehurst. Nor is Lyon’s own tract of woodland any kind of idyll, but rather a “bit of industrial society that lost, over time, to nature”. There, among the trees that have claimed an abandoned railway line, she and her children create a “sacred grove”: cult HQ (an old lean-to adorned with ribbons, sheep’s skulls and a purloined glitterball).

Beyond the railway line, Lyon finds a corporate team-building activity centre, whose hungover inmates she attempts to lure into her grove. “Basically,” she tells them, “I think we’d be better off if we acknowledged that the trees are conscious too.” But the team-builders aren’t “tree people”. Inspired by Aleister Crowley, she instead tries to kickstart things by founding a sex cult. The other mums at the school gates aren’t sure. This is Hay-on-Wye. “That was, apparently, why the swingers’ club in Hay had died out – the inevitability of bumping into each other too soon afterwards.”

Lyon is reluctant to dislodge her tongue from her cheek: “I couldn’t shake the ridiculousness of the whole thing.” But if she is noncommittal about her status as guru, the writing itself is never less than purposeful: scholarly, disarming, occasionally acerbic. Noticing the modishness of “tree-writing”, Lyon regrets that none of it “told me much about the religious quality of a tree, just as the phrenological detail of a human skull failed to say much of the mind within it”. She gamely recruits a local animist to help her “connect shamanically with trees” (the internet is awash with such guides, it seems). In the course of a “shamanic walk”, she learns that the key is to “engage your peripheral vision rather than focusing on any one thing”. There is only so much to be learned from the texture of an oak’s bark or a beech leaf’s serrated edge, however acutely observed.

The iconoclasts of Islamic State have now rolled into Hatra aboard their bulldozers, and the Mesopotamian Green Man is no more. His descendents have been luckier. At Garway and Dore Abbey, Lyon finds she is moved more by the resident Green Men than by the churches themselves, which she experiences as repulsive artefacts of a “grim, repressive faith”. But then, as she acknowledges, the carvings of a tortured Christ were commissioned by the same authorities that sanctioned the apparently pagan symbol that inspires her.

While Lyon might have little time for organised religion (or for organised anything), she is wary of dreamcatchers, “healers” and other manifestations of “woo”. Crowley himself emerges as preposterous. While she struggles to recruit members to her putative cult, Lyon finds no shortage of friends and specialists willing to help her develop what she calls a Green Man-ifesto. They are a likably unorthodox troupe, including G, the builder of the world’s largest depiction of the Green Man (a maze visible on Google Earth), Z, who “ran away with New Age travellers as a teenager” and S, who befriended Pan as a child “and then felt a bit violated when she saw him with a huge erection after taking mushrooms a decade later”.

If the Green Man has a modern human counterpart, Lyon suggests, it is not among the internet’s druids, shamans and occultists, but the American anarcho-primitivist Ted Kaczynski, better known as the Unabomber. Kaczynski’s belief in the alienating influence of technology prompted him to hole up in a Montana cabin from where he mounted a 20-year terrorist campaign that claimed three lives. “An extreme Green Man position,” Lyon concedes drily.

With the exception of Spinoza, whose “God, or Nature” might have been the book’s subtitle, Lyon’s philosophical touchstones are German like her maternal ancestors (one of whom was tried as a witch, we learn). A highlight of the book is her detour to the Black Forest and Trier Cathedral, where some of the earliest depictions of the Green Man are to be found. But it turns out that the Green Man is not a “thing” in modern Germany, for all his Teutonic ancestry and the enduring depth of the German forest. Only in Britain, whose tree cover is among the sparsest in Europe, does the image continue to have any currency as an emblem of a kind of anarchic animism.

Uprooted is less a work of cultural history or countryside writing than a pantheist call to arms. Its subject is the tension between the human and vegetable elements in Green Man imagery. Visiting a junkyard, Lyon admires a yellow Dodge being slowly smothered by brambles. “If the Green Man was consumed, in time, by nature, so too were van and truck and car, dismembered by rain and air and plants in less than a human lifespan.” In Lyon’s eyes it is not so much a memento mori as an image of redemption.

• William Atkins is the author of The Moor; he is writing a book about deserts. To order Uprootedfor £12.79 (RRP £15.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.